IL-7 is a non-redundant cytokine that plays a critical role in regulating the T-cell compartment of the immune system. Due to its ability to maintain the homeostasis of the T-lymphocyte pool and restore it under conditions of lymphopenia, IL-7 is currently under clinical investigation as a potential immune-reconstitution agent for various forms of immunodeficiency, including HIV infection. We previously demonstrated that IL-7 dramatically reduces levels of spontaneous ex vivo apoptosis in both CD4+ and CD8+ T cells derived from HIV-infected patients, while having no significant effect on cells taken from HIV-seronegative individuals. To investigate the effects of IL-7 in an in vivo model of HIV-1 infection, we performed a study of IL-7 treatment during the acute and chronic phases of SIV infection in a well-established nonhuman primate model of AIDS. Previous in vivo studies in macaques chronically infected with SIV demonstrated that IL-7 treatment causes marked increases in circulating CD4+ and CD8+ T-cell counts without significantly increasing the level of viral replication. Likewise, Phase1 clinical trials in patients chronically infected with HIV receiving antiretroviral treatment (ART) have demonstrated that short-term IL-7 treatment is safe and induces transient increases in peripheral CD4+ T-cell counts without inducing sustained increases in HIV replication. Our study included 12 juvenile Rhesus macaques divided into two groups: Group 1 (n=6) was infected with a pathogenic SIV isolate, SIVmac251, and received weekly injections of placebo;group 2 (n=6) was infected with SIV and concomitantly treated with fully glycosylated recombinant macaque IL-7 subcutaneously, once per week for 7 weeks, at a dose of 50 mg/kg. They received no antiretroviral treatment. Multiple clinical, immunological and virological parameters were monitored in all study animals throughout the acute phase of SIV infection and during a follow-up period of 6 months post-infection. Importantly, treatment with IL-7 was safe. IL-7 treatment caused no adverse side effects and did not increase the levels of SIV replication, while it positively affected the immunological profiles of SIV-infected macaques. Specifically, IL-7-treated animals had significantly higher absolute numbers of total circulating CD4+ T cells and CD8+ T cells when compared with untreated animals, and this effect persisted throughout the treatment period. Strikingly, treatment with IL-7 prevented the early depletion of peripheral blood CD4+ T cells that typically occurs during the acute phase of SIV infection. In particular, IL-7-treated animals, unlike untreated control animals, did not experience a depletion of either naive (N) or central memory (CM) CD4+ T cells. This effect was associated with an increase in the intracellular expression of the anti-apoptotic protein Bcl-2, but not with a sustained proliferation of circulating CD4+ and CD8+ T cells, as measured by expression of the nuclear antigen Ki67. This finding suggests that apoptosis reduction was largely responsible for the reduced CD4+ T-cell depletion. This hypothesis was corroborated by studies in peripheral lymphoid tissue (auxiliary lymph nodes) showing that IL-7 treatment reduced the levels of in vivo apoptosis of CM CD4+ T cells, as well as of CM and EM CD8+ T cells. However, the effects of IL-7 were not sustained: The numbers of circulating N and CM CD4+ T cells dropped below pre-treatment levels within 1 month of cessation of treatment. Treatment with IL-7 also induced significant increases in the levels of all subsets of CD8+ T cells (N, CM, EM), which gradually returned to baseline levels after several weeks of treatment interruption. Unfortunately, during the short-term (6 months) follow-up phase, two animals in each study group showed early signs of disease progression with unusual manifestations, such as neurologic and gastrointestinal disease in the absence of significant depletion of circulating CD4+ T cells. This picture is in line with previous descriptions of rapid SIV disease progression (RP) in macaques, reportedly associated with a unique pathogenesis mediated by massive SIV replication in cells of the mononuclear phagocytic lineage rather than CD4+ T cells. Currently we are investigating the gene expression profiles of RP macaque lymphocytes using microarray technology. Preliminary data show differential gene expression at baseline in peripheral blood mononuclear cells taken from RP macaques versus conventional progressors (CP), with upregulation of B-cell-associated genes (immunoglobulins, MHC class II, B-cell activation markers) and downward modulation of T-cell genes, platelet/megakaryocyte genes and the IL-7 receptor gene. Overall, the presence of two RP macaques in each study group suggests that IL-7 had no effect on this dramatic and unusual form of SIV disease. Following the acute phase of SIV infection, IL-7 treatment was restarted during the chronic phase of infection. Escalating doses of IL-7 (from 10 to 50 mg/kg) were restarted at month 8 of SIV inoculation in the 4 remaining animals and treatment was prolonged for 3 months. Re-treatment induced transient increases in CD4+ and CD8+ T cells, which returned to baseline levels or below during this period. Surprisingly, 3 out of 4 IL-7-treated animals progressed to AIDS and were euthanized during the following 4 months. In contrast, all 3 remaining animals in the untreated group were still alive at 18 months of SIV infection and were used for a short-term experiment to investigate the effects of IL-7 treatment on T-cell peripheral homing (see below). IL-7 also plays an important role in peripheral T-cell homing, although the precise tissues targeted and the mechanisms of such homing are largely unknown. Thus, we have started to investigate the ex vivo effects of IL-7 treatment on the expression of a wide panel of tissue homing integrins and chemokine receptors. We have observed that IL-7, in the absence of any other activating stimuli, potently and selectively induces the expression of integrin apha4-beta7 in both CD4+ and CD8+ T cells. This effect, which was observed only at relatively high doses of IL-7 (those reached under conditions of lymphopenia), occurs primarily in phenotypically naive (CD45RA+RO-) T cells. It also is associated with the induction of the functionally activated integrin conformation, as indicated by increased binding activity for the natural integrin ligand, MAdCAM, and for HIV-1 gp120. Further analysis demonstrated that the JAK/STAT and the PI3K/AKT signaling pathways are both involved in upregulating alpha4-beta7 by IL-7. The induction of alpha4-beta7 by IL-7 also was confirmed in vivo in HIV-infected individuals and in SIV-infected macaques after injection of a single dose of IL-7. Preliminary data from late-stage SIV-infected macaques who did not receive virus-suppressing drugs indicate that IL-7 increased the levels of SIV replication and T-cell infiltration in the gut. Altogether, these data suggest a dual role for IL-7 in HIV infection. During acute infection, IL-7 may reduce the levels of apoptosis, thereby preventing the early depletion of CD4+ T cells. Conversely, during late-stage disease, IL-7 may favor homing and proliferation of CD4+ T cells in the gut, providing new targets for HIV replication and fostering the terminal depletion of CD4+ T cells.